1. Field of the Invention
The present invention relates to a capnometer for measuring the concentration of carbon dioxide in expired gases.
2. Related Art
Infrared measurements of carbon dioxide in expired gases are commonly performed with radiation detectors that sense the transmission of radiation associated with the absorption by the carbon dioxide during expiration. The output voltage of the detector is subject to drift for various reasons including the variation in the intensity of radiation from the source and the change in the quantity of radiation due to the contamination of the windows in the sensing part. An apparatus adapted to compensate for such drift is known in the art (see Examined Japanese Patent Publication No. 44614/1985; U.S. Pat. No. 4,067,320).
A problem with this capnometer equipped with the prior art drift compensator in a radiation detector is that the detector uses the rather expensive PbSe. This material features fast response but the device temperature will increase upon continued exposure to an infrared radiation and the decreasing resistance will increase the drift. To avoid this problem, the radiation from the source has to be repeatedly chopped at a frequency, say, 25 ms, interval higher than the respiration frequency. To meet this need, a radiation interrupter and a drive mechanism such as a motor that drives its rotation have been used to detect the quantity of radiation passing through the respiratory gas. However, this has limited the efforts to reduce the overall size of the system and its power consumption while assuring ruggedness. In addition, the prior art system has had the disadvantage of being costly.
Thus, the Applicant developed a capnometer which is capable of compensating for a sensitivity drift of the detector included in the detecting signal and a sensitivity drift, without employing any mechanism for cyclically chopping the infrared light necessary for the PbSe radiation detector (U.S. patent application Ser. No. 08/606,845, now U.S. Pat. No. 5,825,560).
According to the Applicant's proposed technique, the density signal is calculated from the difference between the maximum value of the detection signal during the inspiration phase and the detected signal corresponding to the expiration phase. A reference value is calculated by determining the difference between the maximum value in the inspiration phase and the offset voltage value. However, the Applicant's proposed technique is deficient for the following reasons.
Suppose that once the offset voltage value is detected when the radiation source is turned off momentarily and stored, the reference value is not replaced until the next offset voltage value is detected. This condition results in the following problem for example, as shown in FIG. 7. (1) when light intensity is reduced due to contamination of windows, the difference between the maximum value and the detected signal may contain reduced quantity of detected signal due to the light intensity decrement. So, the measured value may be larger than the actual value as shown by V.times.3 in FIG. 7. (2) As sensitivity decreases, the amplitude of the detected signal caused by changes of CO.sub.2 concentration in the expiration phase is decreased gradually. Since the reference value calculated before the light intensity is reduced is maintained and not replaced until the next maximum value is detected during the next inspiration phase, employing the ratio of such reference value and the density value results in obtaining a measured concentration value which is smaller than actual value and is incorrect as shown by V.times.4 in FIG. 7. (3) In the process where the detected signal is decreasing due to thermal drift or sensitivity variations, as mentioned above, the amplitude of detected signal caused by changes of CO.sub.2 concentration in expiration phase is decreased gradually. This phenomenon occurs due to the tendency of the measured value to become smaller than the actual value. On the other hand, since the maximum values detected in inspiration phases decrease, the difference between the maximum value detected in the inspiration phase and the detected signal detected in the expiration phase reflect the tendency that the measured value becomes larger than the actual value. Therefore, changes of obtained measured values depend on the degree of drift or sensitivity variations which may operate simultaneously (See V.times.5.about.V.times.7 in FIG. 7).
Suppose that replacement of the reference value is performed every respiration by employing the maximum value at each inspiration phase. Under such conditions, the problems noted above may not be resolved as long as the offset voltage value per se drifts.